The present invention generally relates to thermodynamic systems, and more particularly to thermodynamic systems operating according to the Ericsson or Brayton cycle and capable of achieving near-isothermal compression and expansion of a gas by mixing therewith a substantial quantity of liquid.
A refrigeration machine, heat pump, or cooler can be defined as any device that moves heat from a low temperature source to a high temperature sink. Operation of a refrigeration machine requires an input of energy, usually thermal, mechanical or electrical. Depending on the specific need, the heat absorbed in the low temperature source can be utilized to provide cooling, or the heat rejected to the high temperature sink can be used to provide heating, or both may be utilized simultaneously. As an example, for a typical household refrigerator the low temperature source is the space inside the refrigerator and the high temperature sink is the air in the room where the refrigerator is placed. Electrical energy is typically used to operate the system.
With the exception of a few niche applications, virtually all refrigeration machines operate on the vapor-compression (V-C) cycle. Common examples include home and automobile air conditioners, domestic and industrial food refrigeration, commercial comfort cooling, industrial process cooling, and many others. The traditional refrigerant fluids used in these machines contain compounds that result in ozone depletion if they escape into the upper atmosphere. These ozone depleting refrigerants are in the process of being phased out and eventually banned. However the new refrigerants, while not posing a risk to the ozone layer, are very potent greenhouse gasses. Other refrigerants that don't pose a substantial environmental risk have other drawbacks, such as being flammable or toxic. One such example is ammonia, which is an excellent refrigerant from a system performance perspective, but is highly toxic. There is a great need and much work is being done to develop and commercialize practical refrigeration systems that do not require the use of environmentally hazardous refrigerants.
The reverse Ericsson cycle is an alternative refrigeration cycle capable of operating with benign refrigerants, such as air, argon, xenon, and helium. The Ericsson cycle combines four thermodynamic processes. For an ideal cycle that uses a gas as the working material, the processes are isothermal (constant temperature) compression, constant pressure heat rejection from the high pressure stream to the low pressure stream, isothermal expansion, and constant pressure heat addition to the low pressure stream from the high pressure stream. A system that approximates these processes can be termed an Ericsson device or machine. The Ericsson cycle has several notable advantages. For example, the cycle is thermodynamically reversible, meaning that its coefficient of performance (COP) is theoretically the same as the Carnot COP, which is the maximum efficiency any refrigeration machine can achieve while operating between given temperatures. Another advantage of the Ericsson cycle is that it can use fluid refrigerants that pose no or low environmental risk. Virtually any gas can be used as the working fluid, including the aforementioned air, argon, xenon, and helium as well as other readily available gases such as carbon dioxide.
The principle difficulty of implementing a practical device that operates in a manner substantially similar to the Ericsson cycle is the requirement for isothermal or near isothermal compression and expansion of the working fluid to achieve a reasonable efficiency. When a gas is compressed, the temperature of the gas increases. To keep the temperature of the gas constant during compression, the gas must be cooled while it is compressed. In practice, isothermal compression of a gas is extremely difficult to achieve because, for practical compression machines, the area available for heat transfer is very small and the compression process occurs very quickly. Slowing down the compression process or increasing the surface area for heat transfer leads to very large, impractical, and expensive machinery.
U.S. Pat. No. 4,984,432 to Corey discloses an Ericsson cycle machine that uses liquid ring compressors to compress and expand a gas-liquid mixture. However, several disadvantages are believed to exist with this machine as disclosed. First, liquid ring compressors have difficulty producing large pressure differentials, which can result in small volumetric capacities and necessitate large equipment to achieve relatively small cooling capacities. Liquid ring compressors also exhibit low efficiencies due in part to high viscous (fluid friction) losses, resulting in tremendous degradation of performance. Furthermore, the power required to pump the liquid through the heat exchanger loops is substantial, with no means disclosed to recover this power. Another shortcoming is that the liquid ring is simultaneously in substantial thermal contact with both the inlet and outlet gas streams, which has the undesirable effect of preheating the suction gas on the compression side and precooling the inlet gas on the expander side and results in higher compression work and lower expander work recovery, respectively. In any event, a thermodynamic analysis of the cycle is not presented in the Corey patent, and attempts to test the disclosed Ericsson cycle machine have failed to achieve a net heat pumping effect.